The tension compounds when the family moves to a small town deep in the country, forcing Molly and Michael to cancel the summer extracurricular programs they signed up for.

Molly finds Heather one night in the ruins of a house near a pond, talking to a ghost child named Helen.

Michael and Molly visit the town library and learn that Helen Elizabeth Harper died one hundred years before.

Her mother and stepfather died in a fire; Helen escaped the blaze only to panic and run into a nearby pond, where she drowned.

One afternoon Molly returns home to find a note from Dave and Jean saying they went shopping in Baltimore, leaving Heather unsupervised.

The girls take refuge in the ruins, but the floor collapses beneath them, plunging them into the cellar where they discover the skeletal remains of Helen's mother and stepfather.

Helen likewise accidentally started the fire that killed her parents, making her the only person who understood Heather's guilt.

On 28 August 2014, actress Sophie Nélisse stated on her Twitter account that she will act in a film adaptation of the book as Molly.

[4] On 12 September 2014, Variety announced that financing and cast were in place and principal photography would commence on the production of a film adaptation of Wait Till Helen Comes.

Author Mary Downing Hahn appears in the film in a speaking role, mentioning to one of the producers that as a little girl she had always wanted to be an actress in movies.